In many medical conditions, it is desirable to kill specifically a selected population of cells, for example, diseased or abnormal cells such as tumor cells, parasitic organisms, certain subgroups of cells of the immune system that may be responsible for autoimmune diseases or responsible for rejection of organ and tissue grafts, or cells infected by microorganisms or viruses (such as HIV, the infectious agent for acquired immunodeficiency syndrome). One strategy that is under current investigation is to use specific antibodies, preferably monoclonal antibodies, or other specific cell-binding molecules, to deliver toxic agents to specific cells in order to kill them selectively. For this approach to be successful, it is desirable that the toxic agent be extremely potent so that delivery of a few molecules to the target cell will be sufficient to kill the cell. However, the toxic agent must exhibit low toxicity towards non-target cells so that only the targeted cells will be killed.
Previously, several monoclonal antibodies have been identified that show specificity for tumor cells and many of these are summarized by Frankel et al., Ann. Rev. Med. 37, 125-142 (1986). Such monoclonal antibodies, and others that may be developed in future work, may be suitable candidates to be used to deliver toxic agents specifically to tumor cells. In principle, monoclonal antibodies can be made that can bind specifically to any particular antigen, including such antigens as may be concentrated on, or found exclusively on, a particular population of cells that it would be desirable to kill, and these antibodies can be linked to toxic agents in order to deliver the toxic agents to the specific cell population.
There are several known cytotoxic lectins such as ricin, abrin, modeccin, volkensin, viscumin and Shigella toxin which kill eucaryotic cells very efficiently. These cytotoxic lectins contain two different types of subunits. One type of subunit, designated the A-chain, exhibits cytotoxic activity by catalytic inactivation of ribosomes, while the other type of subunit, designated the B-chain, is a lectin portion with a recognition capacity, see "The Lectins: Properties, Functions and Applications in Biology and Medicine" (Liener et al., eds.) Academic Press, N.Y. (1986). Since molecules or receptors to which lectins will bind are ubiquitous on cell surfaces, the cytotoxic lectins are non-discriminating, and hence non-selective in their cytotoxicity. This characteristic greatly limits their utility for killing selected diseased, infected or abnormal cells such as tumor cells.
A high resolution X-ray structure for one of these cytotoxic lectins, ricin, has been published by Rutenber et al., Nature, 326, 624-626 (1987) and Montfort et al., J. Biol. Chem. 262, 5398-5403 (1987). These works show that the ricin B-chain has two galactose-binding sites that are well separated. Previously, cytotoxic lectins have been cleaved into their two types of subunits and the A-chains alone linked to monoclonal antibodies to provide toxic conjugates having selective cytotoxic activity. But such conjugated products display significantly reduced toxicity as compared to intact lectins. Vitetta et al., Science 219, 644-650 (1983); Vitetta et al., Transplant. 37, 535-538 (1984); Thorpe et al. in "Monoclonal Antibodies '84: Biological and Clinical Applications" (Pincheva et al., eds.) pp. 475-512, Editrice Kurtis s.r.l. (1985); Pastan et al., Cell 47, 641-648 (1986) and Frankel et al., Ann. Rev. Med. 37, 125-142 (1986) provide summaries of previous work using this approach. The A-chain of a cytotoxic lectin has also been linked to other specific cell-binding proteins, such as epidermal growth factor (Cawley et al., Cell 22, 563-570 (1980) and Simpson et al., Cell 29, 469-473 (1982)) and transferrin by Raso et al., J. Exp. Med. 160, 1234-1240 (1984). for example. These conjugates exhibited specific cytotoxic activity but also displayed significantly reduced toxicity as compared to the whole cytotoxic lectin.
Previously, conjugates have been made by covalently linking a whole cytotoxic lectin to monoclonal antibodies. Whole ricin has been linked to monoclonal antibodies by Youle et al., Proc. Natl. Acad. Sci. (USA) 77, 5483-5486 (1980); Neville et al., U.S. Pat. No. 4,539,457 and U.S. Pat. No. 4,440,747; Houston et al., Canc. Res. 41, 3913-3917 (1981); Thorpe et al., Nature 297, 594-596 (1982); Vallera et al., Transplant. 36, 73-80 (1983): Vallera et al., Science 222, 512-515 (1983); Quinones et al., J. Imm. 132, 678-683 (1984); Leonard et al., Canc. Res. 45, 5263-5269 (1985); Marsh et al., Biochem. 25, 4461-4467 (1986) and Goldmacher et al., J. Biol. Chem. 262 3205-3209 (1987). Whole ricin has also been linked to other cell-binding molecules such as epidermal growth factor by Herschman, Biochem. Biophys. Res. Commun. 124, 551-557 (1984) and monophosphopentamannose by Youle et al., Proc. Natl. Acad. Sci. (USA) 76, 5559-5562 (1979).
Conjugates made by linking a whole cytotoxic lectin, such as whole ricin in the examples cited above, to a specific cell-binding agent, exhibit the high cytotoxicity of the cytotoxic lectin. However, such conjugates only exhibit selective killing of the cells targeted by the monoclonal antibody or other cell-binding agent when the conjugates are incubated with cells in the continuous presence of high concentrations of a sugar, for example lactose or galactose, that can bind to the binding sites of the cytotoxic lectin. This is because the binding of the sugar to the binding sites of the lectin portion reduces the non-selective binding of the cytotoxic lectin to cell surfaces. The non-selective toxicity of these conjugates in the absence of high concentrations of sugar greatly limits their usefulness for killing selected cells, such as diseased or abnormal cells, in vivo. However, successful experiments have been performed with a human solid tumor in a nude mouse model system, where a ricin conjugate was injected intratumorally in a solution containing lactose and at the same time the animals received intravenous (i.v.) injections of solutions of lactose (Weil-Hillman et al., Canc. Res. 45, 1328-1336 (1985) and Canc. Res. 47, 579-585 (1987)).
Conjugates with a whole cytotoxic lectin such as whole ricin may be used in vitro in the presence of lactose or galactose to kill specific populations of cells. Filipovich et al., The Lancet. Mar. 3, 1984, pp. 469-472 report the clinical use of conjugates of whole ricin linked to monoclonal antibodies that were specific for human T cells for the ex vivo treatment of human bone marrow to prevent graft-versus-host disease.
Vitetta et al., Proc. Natl. Acad. Sci. (USA) 80, 6332-6335 (1983) and McIntosh et al., FEBS Lett. 164, 17-20 (1983) report the use of ricin B-chain to increase the potency of conjugates of monoclonal antibody linked to ricin A-chain. However, this approach may have limited utility in vivo because there is a chance that whole ricin will form, once the B-chain is mixed with the A-chain, which could then exhibit high toxicity towards non-target cells.
It would be desirable to block permanently the binding sites of cytotoxic lectins in a stable way, and thereby eliminate the ability of the cytotoxic lectins to kill cells without regard to selectivity, while preserving their ability to kill when, and only when, the blocked cytotoxic lectins are linked to a cell-binding molecule or substance which is selective for particular cells or cell populations. Selective killing of particular cells is, therefore, dependent on the cell-binding agent.
In one approach to interfere with the binding of cytotoxic lectins to cells, Sandvig et al., Eur. J. Biochem. 84, 323-331 (1978) treated abrin and ricin with various chemical reagents specific for various chemical functions that are found in proteins and glycoproteins, and demonstrated that acetylation of the cytotoxic lectins, primarily at tyrosine residues, with the reagent N-acetylimidazole, resulted in a 94% reduction in cell-binding activity of abrin and an 88% reduction in cell-binding activity of ricin together with a 94% reduction in toxicity of both lectins. Acetylation of tyrosine residues in wheat germ agglutinin and in lentil lectin, with concomitant loss of agglutination activity, has been, reported by Rice et al., Biochem. 14, 4093-4099 (1975) and Vancurova et al., Biochim. Biophys. Acta 453, 301-310 (1976), respectively. Acetylated ricin has been used by Leonard et al., Canc. Res. 45, 5263-5269 (1985) to form a conjugate with an anti-T cell monoclonal antibody. This preparation of acetylated ricin showed a 10-fold reduction in non-selective toxicity as compared to ricin. This toxicity was reduced further by 10-fold to 100-fold in the presence of 0.1M lactose suggesting that the binding ability of the ricin was, at best, only partially eliminated by acetylation. Vitetta. J. Imm. 136, 1880-1887 (1986) described the use of oxidation to alter the ricin B-chain so that its ability to bind as a lectin was greatly reduced. However, the oxidized B-chain of ricin was drastically altered by the oxidizing agent such that it could no longer associate with the ricin A-chain and therefore no whole ricin could form.
Another approach that has been taken with the goal to eliminate the binding of lectins to cell surfaces, has been to physically or sterically hinder their binding ability by covalent linkage of a large molecule to the cytotoxic lectins. Thorpe et al., Eur. J. Biochem. 140, 63-71 (1984) described conjugating a monoclonal antibody to intact ricin, then fractionating the product by affinity chromatography to isolate in 20% yield a fraction in which the monoclonal antibody by chance blocked the oligosaccharide-binding sites of the ricin thereby diminishing the capacity of the conjugate to bind non-specifically to cells.
Alternative approaches to reduce the non-selective cytotoxicity of cytotoxic lectins have been to attempt to interfere with binding site(s) of the lectin in more specific ways. In these approaches, molecules that can focus the chemical modification of lectins to the binding sites of the lectins have been used. Such molecules can be modified sugars or compounds containing carbohydrate that can react chemically with lectins at the binding site(s) of the lectins, thus blocking the binding site(s) of the lectin. In one approach, concanavalin A was treated with a photoactivatable arylazido derivative of mannose which binds specifically to the concanavalin A binding sites, followed by exposure to ultraviolet light to form a covalent bond between the concanavalin A and the sugar derivative, as described by Beppu et al., J. Biochem. 78, 1013-1019 (1975). The product retained binding activity at two of its four binding sites and displayed reduced haemagglutinating activity. Similar results were recorded by Fraser et al., Proc. Natl. Acad. Sci. (USA) 73, 790-794 (1976) using succinylated concanavalin A. and by Thomas, Meth. Enz. 46, 362-414 (1977) in an analogous procedure. A similar procedure has also been employed using ricin and a photoactivatable derivative of galactose as described by Houston. J. Biol. Chem. 258, 7208-7212 (1983). The product was found to be 280 times less toxic toward cells than untreated ricin, although the A-chain alone showed full activity in the inhibition of protein synthesis in cell lysates.
Photoactivatable derivatives of complex glycopeptide ligands having a higher affinity for lectins than simple monosaccharides and disaccharides have been described by Baenziger et al., J. Biol. Chem. 257, 4421-4425 (1982). The lectins concanavalin A, ricin, and lectin from liver were treated with appropriate photoactivatable derivatives of glycopeptides, and then exposed to light to form a covalent linkage between the lectin and the glycopeptide derivative. However, only 1-2% incorporation of the glycopeptide derivative was achieved in each case. There was, apparently, no attempt to determine the efficacy of the labelling of the lectins through measurements of either cytotoxicity (in the case of ricin) or of haemagglutination (in the case of concanavalin A).
The photoaffinity reagents described by Baenziger et al., J. Biol. Chem. 257, 4421-4425 (1982) were made by covalently linking a photoactivatable group to the peptide portion of the glycopeptide ligand. Lee et al., Biochem. 25, 6835-6841 (1986) disclose a photolabeling reagent made by covalently linking a photoactivatable group to the C-6 position of a galactosyl residue of a glycopeptide ligand derived from asialofetuin. Lee et al. (cited above) used this photoactivatable affinity labeling reagent to react with the binding site(s) of lectin from liver. However, less than 1% of the photoactivatable glycopeptide ligand could be incorporated into the lectin from liver even under conditions of binding site excess.
It is clear that the B-chain of ricin, abrin, and the like, which is the lectin portion of these cytotoxic lectins, has two functions. One of these functions relates to the binding capability of the molecule as a lectin (Olsnes et al., Biochem. 12, 3121-3216 (1973) and Robertus et al., J. Biol. Chem. 259, 13953-13956 (1984)), which enables these cytotoxic lectins to bind to lectin receptors on the surface of cells. The second function of the B-chain can be called a transport function, where the B-chain participates in, or facilitates, the process of translocation of the A-chain across a membrane of the cell into the cytoplasm of the cell to which the cytotoxic lectin is bound (Youle et al., Proc. Natl. Acad. Sci. (USA) 76, 5559-5562 (1979) and Youle et al., J. Biol. Chem. 257, 1598-1601 (1982)). Up until the present invention, there was no convincing evidence that these two functions could be uncoupled. Indeed, there are reports that these two functions of the B-chain, binding as a lectin and transport, cannot be separated from one another (Youle et al., Cell 23, 551-559 (1981)). One reason for the lack of convincing evidence that these two functions can be uncoupled is the poor yield and variability in end results in the attempts to block or to interfere with the binding sites of the cytotoxic lectins, either by using the approach of targeting chemical modification to the binding site(s) of the lectins with photoactivatable derivatives of compounds containing carbohydrate and simple sugars, or by interfering with the binding sites by steric hindrance or by non-specific chemical modification of the cytotoxic lectins.
Lee et al., J. Biol. Chem. 258, 199-202 (1983) and Lee et al., Biochem. 23, 4255-4261 (1984) have shown that a lectin from liver has a much higher affinity for certain branched oligosaccharides than for non-branched oligosaccharides and monosaccharides. Baenziger et al., J. Biol. Chem. 254, 789-795 (1979) describe the complete structure of the complex N-glycosidically-linked oligosaccharides of fetuin, the major glycoprotein in fetal calf serum, and the proposed structure for the branched oligosaccharide is represented in FIG. 1(A). This structure was largely confirmed by the evidence reported by Nilsson et al., J. Biol. Chem. 254, 4544-4553 (1979) except for one difference at a mannose branch point as represented in FIG. 1(B). More recent analyses by Takasaki et al., Biochem. 25, 5709-5715 (1986) and Townsend et al., Biochem. 2, 5716-5725 (1986) have shown that the branched oligosaccharides of fetuin are heterogeneous, but that the major species is that represented in FIG. 1(B), consistent with the structure reported by Nilsson et al., J. Biol. Chem. 254, 4544-4553 (1979). It was Baenziger et al., J. Biol. Chem. 254, 9795-9799 (1979), who determined that asialoglycopeptides, derived by removing sialic acid from glycopeptides derived from fetuin, bound to ricin with a high affinity, having association constants of about 10.sup.7 M.sup.-1.
The work of Rutenber et al., Nature 26, 624-626 (1987), Montfort et al., J. Biol. Chem. 262, 5398-5403 (1987), Frenoy. Biochem. J. 240, 221-226 (1986) and Houston et al., J. Biol. Chem. 257, 4147-4151 (1982) suggest that ricin binds galactose at two distinct sites. There is, however, no direct evidence about whether ricin can bind more than one complex N-glycosidically-linked glycopeptide ligand, such as may be derived from asialofetuin (e.g., Baenziger et al., J. Biol. Chem. 254, 9795-9799 (1979)). Two galactose-binding sites on a ricin molecule may bind to two different galactose moieties of a single complex glycopeptide ligand, thus accounting for the high affinity of the association between ricin and such glycopeptide ligands.
The variability in end results using prior art blocking agents for the binding sites of lectins is eliminated by the present invention which provides effective and reproducible reduction of the non-selective action of lectins without excessive reduction of their cytotoxic or other desired properties when they are linked to a cell-binding agent.
The CD19 antigen, originally designated as the B4 antigen, is a 95 kd glycoprotein, which is B cell lineage-restricted within the hematopoietic system. B4 was defined originally by a monoclonal antibody clone 89B (=anti-B4; Nadler et al., J. Imm. 131, 244-250 (1983)) and the anti-B4 antibody has been accepted by the WHO in Geneva as a standard to define human B lymphocytes. Other laboratories have subsequently produced monoclonal antibodies against the B4 antigen and the antibodies and the antigens recognized thereby have been grouped into a cluster called CD19, ("Leukocyte Typing II, Human B Lymphocytes" (Reinherz et al., eds.) Vol. 2. Chaps. 12 and 13, pp. 155-175. Springer-Verlag (1986)) with anti-B4 being the prototype for this cluster.
The major diseases associated with B cells or pre-B cells where the CD19 antigen is expressed are leukemias and lymphomas. Because the CD19 antigen appears on early pre-B cells and lasts through all stages of B cell ontogeny (Freedman and Nadler, Sem. Onc., 14, 193-212 (1987)) it is expressed in all pre-B cell and B cell neoplasia, i.e. in 95% of all non-T cell ALL (acute lymphocytic leukemia), in 100% of all B-CLL (chronic lymphocytic leukemia of B cell origin), in all types of B cell-derived non-Hodgkin's lymphomas (Burkitt's lymphomas, nodular lymphomas and diffuse lymphomas) in hairy cell leukemias and in Waldenstrom's macroglobulinemia. (Freedman and Nadler. Sem. Onc., 14, 193-212 (1987)). Additionally, although the multiple myeloma cell does not express the CD19 antigen, the clonogenic myeloma cell is CD19-positive, i.e., does express the CD19 antigen.
Although leukemias and lymphomas are very responsive to current conventional and high-dose chemotherapy, the actual cure rate is not very high. (DeVita et al. in "Cancer: Principles and Practice of Oncology" (DeVita et al., eds.) 2nd ed. (1985)). The treatment (radiation and chemotherapy) is limited largely by its non-specific toxic side effects. Accordingly, treatment with a medicament having increased specificity and much higher cytotoxicity, i.e. it kills cells at very low concentrations, is highly desired.
For example, a treatment for some leukemia and lymphoma patients is autologous bone marrow transplantation. In this procedure (Takvorian. T. et al., New Eng. J. Med. 316, 1499-1505 (1987)), bone marrow is removed from the patient and treated ex vivo to remove cancerous cells. The purged marrow is reinfused into the patient after his body has been treated by chemotherapy and total body irradiation. The current ex vivo purging of the marrow consists of three cycles of treatment of the mononuclear cells with monoclonal antibodies and baby rabbit complement. This treatment modality is very time consuming and suffers from the problem of variability in complement activity. Every new batch of complement must be screened for activity and non-specific toxicity. Many batches are rejected. A treatment that could achieve the same amount of purging in a single treatment cycle and does not have to be tested from batch to batch, because it shows consistent activity and specificity, would greatly simplify the current treatment.
Human B lymphocytes are also involved in many autoimmune diseases, such as lupus erythematosus, rheumatoid arthritis, myasthenia gravis, multiple sclerosis, immune thrombocytopenia and other autoimmune conditions. In these diseases, autoantibodies are produced by the patient.
Today these diseases are treated with general immunosuppressing drugs such as steroids, cyclophosphamide, rheumatrex (methotrexate) and imuran, which cause side effects such as increased susceptibility to infections. Again, a medicament and treatment that would be more specific and cause less side effects is greatly desired.
A third condition where the elimination of human B cells would be beneficial to patients is in the organ transplant setting. For example, a large number of patients awaiting renal transplantation are sensitized against HLA antigens and produce anti-HLA antibodies, leading to rejection of the transplanted kidney. Currently this reaction is suppressed by the administration of non-specific cytotoxic drugs, such as prednisolone and cyclophosphamide. However, this treatment is not very effective and in addition causes general immunosuppression. A more effective drug and most importantly, a drug that is very specific for the antibody-producing B cells which therefore would not cause all the side effects is much desired.
There are other diseases where increased amounts of immunoglobulin are produced, and which therefore are candidates for medicaments and methods of treatment that deplete B cells. These include conditions like idiopathic thrombocytopenic purpura and hemolytic anemias. It would be desirable to be able to destroy the abnormal B cell clones by treating the patients with an anti-CD19 medicament.
Another desired medicament is one that would block the humoral immune response to foreign proteins. When foreign proteins are administered to humans many develop antibodies against these foreign proteins, which diminishes or abolishes the beneficial effect of the proteins.
Additionally, a reagent that can be used as a research tool in vitro, when it is necessary to have highly purified populations of T cells, of granulocytes (neutrophils, eosinophils and basophils) or of monocytes is much desired. Typically such cell populations will be contaminated with B cells, even if they have been isolated by flow cytometry or panning. Treatment with an anti-CD19-specific cytotoxic reagent would eliminate B cells more thoroughly, would be easier than flow cytometry and could be used on large numbers of cells. Pure cell populations are necessary to study their characteristics such as lymphokine production, reaction and sensitivity to different stimuli and cell-specific expression of certain gene products.
The CD33 antigen, originally designated as the My9 antigen, is a 67 kd cell surface glycoprotein (gp 67) expressed exclusively on the surface of granulocytes, monocytes and their precursor cells in normal human bone marrow. CD33 was defined originally by antibody anti-My9 produced by hybridoma clone 906 (Griffin et al., Leuk. Res. 8, 521-534 (1984)). subsequently have made monoclonal antibodies which react with this antigen and the antibodies and the antigens recognized thereby have been grouped together in the cluster designation CD33.
In one study (Griffin et al., Leuk. Res. 8, 521-534 (1984)). 84% of 98 cases of AML (acute myelocytic leukemia) tested reacted with anti-My9 monoclonal antibody by immunofluorescence. Only small numbers of other types of leukemic cells reacted with this antibody. The finding of a high percentage of AML cells reactive with this antibody has been confirmed in many other studies, including a multi-institutional national trial conducted by the Cancer and Leukemia Group B (Griffin et al., Blood 68, 1232-1241 (1986)) in which it was demonstrated that the antibody reacted with leukemic cells from 71% of 196 cases. The antibody has been shown to be useful in making the diagnosis and confirming the diagnosis of AML, and it is distributed by Coulter Immunology. Hialeah, Fla.
As is the case for many human malignancies, the neoplastic cells in individual cases are heterogeneous. This is particularly true for acute myeloblastic leukemia where only a small fraction of cells is capable of proliferation and the majority of cells are nonproliferating cells which accumulate in the bone marrow and other organs. A current belief is that the proliferative cells act as stem cells for maintaining the leukemic state. A review of AML can be found in Griffin and Lowenberg, Blood 68, 1185-1195 (1986). Importantly. in a study of 20 patients with AML, it was shown that the CD33 antigen was expressed on the proliferative. clonogenic AML cells from at least 18 of the 20 patients tested. This study was done using complement lysis. This study suggests that the CD33 antigen is expressed at all stages of differentiation of myeloblastic leukemia cells. Thus, the removal of CD33-positive leukemic cells could eliminate the clonogenic (proliferative) fraction of leukemic cells. CD33 antibodies could form the basis for a rational approach to purging leukemic cells in this disorder.
AML is the most deadly leukemia and no efficacious treatment is available to date (Champlin & Gale. Blood 69, 1551 (1987); DeVita et al., supra and "Cancer Facts and Figures" (American Cancer Society) (1988)). Most patients will die within 2-3 years of prognosis of their disease, with 50% of the deaths occurring within 6 months (DeVita et al., supra).
From this data it is clear that the current therapies are not effective for treating AML. The most effective therapy is chemotherapy (Champlain and Gale, Blood 69, 1551 (1987) and DeVita et al., supra), which is severely limited by its toxic side effects and by the induction of drug resistance.
The CD56 antigen, originally designated as the N901 antigen or NKH-1 antigen (Knapp et al., Int. J. Canc. 44, 190-191 (1989), is a 200,000-220,000 molecular weight glycoprotein, which is considered a pan-natural killer (NK)-associated cell surface marker. The N901 antigen was defined originally by a monoclonal antibody produced by fusing NS-1 myeloma cells with spleen cells of a mouse immunized with human chronic myelogenous leukemic cells (Griffin et al., J. Imm. 130, 2947-2951 (1983)). The anti-N901 antibody is available commercially from Coulter Corporation under the name NKH-1. Another antibody has been produced against the N901 antigen and is referred to as anti-NKH1a (Hercend. et. al., J. Clin. Invest. 75, 932-943 (1985)).
CD56 is a specific marker for neuroendocrine-type tumors with a concordance rate of expression of nearly 100% (Gazdar, 2nd Ann. Symp. Coulter Imm., p. 26, CA (1986)). Neuroendocrine tumors include small cell lung cancer (SCLC), pituitary adenomas, medullary thyroid carcinoma, carcinoids, islet cell tumors, neuroblastomas and pheochromocytomas. Expression of CD56 on SCLC specimens has been documented with 100% concordance on both cell lines derived from SCLC tumors and primary biopsies from SCLC patients (Gazdar, supra: Koros et al., Proc. Third Intl. Work. Conf. Hum. Leuk. Diff. Antigens (1986) and Doria et al., Canc. 62, 1939-1945 (1988)). NKH-1 has been grouped into cluster 1 of SCLC determinants by the SCLC antigen workshop (Beverly et al., Lung Canc. 4, 15-36 (1988).
There are approximately 155,000 new cases of lung cancer and 142,000 deaths from this disease per year in the United States ("Cancer Facts and Figures" (American Cancer Society) pp. 8-9, (1989)). SCLC will account for 20-25% of these cases. The current treatment for SCLC primarily involves intensive combination chemotherapy and radiotherapy (Jackson and Case, Sem. Onc. 13, 63-74 (1986) and Postmus et al. in "High-Dose Chemotherapy for Small Cell Lung Cancer in Lung Cancer: Basic and Clinical Aspects" (Hansen. ed.) (1986)). While over 75% of patients will demonstrate an objective tumor response to therapy, the median survival of all patients from the onset of therapy is only 11 months. Limiting factors in the use of conventional chemotherapy for treatment is the degree of non-specific toxicity and the development of drug-resistant tumor cells.
An additional form of treatment for SCLC involves autologous bone marrow transplantation. In this procedure, bone marrow is removed from the patient and is cryopreserved, and then the body is purged of all cancer cells by large doses of chemotherapy and of totalbody irradiation. After the body has been purged, the preserved bone marrow is reinfused. However, in many SCLC patients the bone marrow also contains cancerous cells and has to be treated ex vivo (Mabry et al., J. Clin. Invest. 75, 1690-1695 (1985)).